The present invention relates to methodology of monitoring the state of an exposure process step in the manufacture of a semiconductor device. More particularly, but not exclusively, this invention relates to a method and apparatus for monitoring an exposure process state by measurement of the shape of a resist pattern that is formed through exposure.
FIG. 2 shows a flow of a prior known lithography process. A resist pattern is formed on or above a substrate, such as a semiconductor wafer, by depositing a resist film made of photo-sensitive material to a predetermined thickness, then using an exposure machine to perform scale-down exposure of a mask pattern (at step 2050), and thereafter developing it (2051). The resist pattern thus formed is next subjected to size check by use of a scanning electron microscope (SEM) with size measurement functions, such as length-measuring SEM or CD-SEM (at step 2052). Exemplary contents of the processing in prior art length-measure SEMs are as follows: performing size measurement after having acquired an electron beam image of a region that includes certain portions that are under requirements for strict management of size accuracy; determining whether the size measured satisfies a reference or standard value (at step 2053); and, modifying or correcting, if it fails to satisfy, the exposure amount of exposure machine (at step 2054, where an exposure correction amount is given as ΔE). For example, in the case of a positive resist, if the resist size is found to be too large, then increase the exposure amount; if too small then reduce the exposure amount. In many cases, such increment or decrement of the exposure amount is determined based on an operator's own experience and feelings.
FIG. 3 shows a typical relationship of a resist pattern and a film pattern after etching (quoted from a research paper of 98th Meeting of the 132nd Committee for Industrial Application of Charged Particle Beams by Japan Society for the Promotion of Science (JSPS), titled “Electron Beam Testing Handbook” at p. 255). Generally, the shape of the resist pattern and the shape of the etched film pattern exhibit a constant relationship as far as the etching conditions are the same. In order to obtain the intended film pattern with a predefined shape, the resist pattern also is required to have a predefined shape.
In the event that production lines are built up for fabrication of a new type of semiconductor substrates, the so-called “condition setup” works are carried out prior to introduction of product wafers. These works are for finding out the exact focussing position and proper exposure amount with which a predetermined resist pattern shape is obtainable through the steps of, for example, making pattern-printed wafers while varying a focussing position and an exposure amount on a per-shot basis (i.e., per exposure unit)—generally, such wafers are called the focus and exposure matrix (FEM) wafers—and then performing size measurement of a resist pattern of each shot, followed by the steps of cutting a wafer into portions and analyzing their cross-sectional shapes.
By this work operation, the optimal exposure amount and the optimum focusing position are determined. Then, exposure of product wafers is performed based on such the conditions. However, various process variations can take place over time. Examples of the process variations include, but not limited to, a drift of several types of sensors in an exposure machine, a change in resist photosensitivity, and a change in post exposure bake (PEB) temperature. These unwanted variations would in some cases result in the lack of an ability to obtain any intended resist pattern with an appropriate shape under the conditions determined by the condition setup works. Detecting this is an important role of the above-noted size measurement (at step 2052 of FIG. 2). In the prior art, attempts have been made to compensate for such process variability by modification or amendment of the exposure amount while regarding the size as a barometer of process variations.
It is noted that JP-A-2003-173948 discloses therein a technique for estimating a deviation amount from the proper criteria of an exposure process to be monitored, by using feature quantity obtainable from an acquired secondary electron signal to thereby create model data for correlating exposure conditions with a SEM image, and also by comparing the feature quantity obtainable from the secondary electron signal to the model data stated above.
In the prior art, in order to detect a process variation and then take necessary corrective action, a method is employed for using a length-measuring SEM to examine more than one size value such as a line width or the like and for correcting the exposure amount if the size value fails to satisfy a reference value.
In recent years, the pattern scaling has much progressed resulting in an increase in integration density of semiconductor devices. As semiconductor patterns shrink, ultra-high image resolution techniques using advanced masks such as Levinson phase shift masks are more widely used in currently established semiconductor fabrication processes. This results in a drastic decrease in allowable variability of the exposure amount and focus position. Today, it is difficult to maintain the process of interest within a proper range by mere use of the exposure correction technique. For example, it is required to control the process parameters in a way which follows: for 65-nm nodes, the permissible variability of the exposure amount falls within a range of 8 to 10% or less while letting the allowable variability of the focus position be about 200 to 300 nanometers (nm) or less. To achieve this, a need is felt to provide the information that quantitatively indicates process variations—that is, accurate quantization of variations to exactly specify the degree of a deviation of the exposure amount in the unit of millijoule (mJ) and also the degree of an offset of the focus position in unit of nm.
The above-noted prior art suffers from a risk that a variation of the focusing position can be overlooked (because such focus position variation does not always accompany with size variations). Another risk faced with the prior art lies in the inaccuracy of detection of a deviation of the exposure amount (because size variations can occur due to focus position offset also). Furthermore, it is apparent that even in cases where the focus position should be amended, any resist pattern with the proper shape is not obtainable in some cases because of the execution of such exposure amendment. Thus, it is unlikely that the prior art achieves appropriate maintenance of exposure process.